The present invention relates to improvements in manually-operated electric motor starter and overload protection systems, and particularly to the elimination of all electromagnetic circuitry therein and resultant improvement in safety and reliability.
Conventional electric motor starter systems characteristically feature solenoid-operated switches in the main power conduits. Operator control is accomplished by means of relatively low current, manually-operated electric switches which selectively energize or de-energize the solenoids through an electromagnetic actuating circuit. Each main power conduit has a conventional overload sensing relay which de-energizes the electromagnetic circuitry if an overload is sensed.
The problems with such electromagnetically-actuated motor starter systems are twofold. First, particularly with respect to electric motors which are not normally continuously running but rather require frequent switching for momentary operation, the electromagnetic components tend to wear and deteriorate too rapidly, causing excessive down time and repairs. Second, deterioration of the electromagnetic components such as the solenoid and overload relays tends eventually to result in current leaks from the main motor power circuitry into the electromagnetic control circuitry and thence to the manually-operated control switches, resulting in severe hazards to the operator from electric shocks.
Switching systems such as those shown in Willheim U.S. Pat. No. 2,078,784 and Spencer U.S. Pat. No. 2,562,847 have been devised utilizing actuating circuitry which combines electromagnetic and pneumatic principles. However such systems merely reduce reliability and add expense by retaining the electromagnetic circuitry and adding a second pneumatic circuit to it. Moreover when pneumatic actuators are used, as in Willheim, the application of gas pressure conventionally opens the switches rather than closing them, causing the switches to be closed if the source of pressurized gas fails. This practice is not acceptable for application to electric motor control systems since the failure of power in the actuating circuit must result in stoppage of the motor for safety reasons. Further, such systems as that shown by Spencer are not adaptable for application to continuously running motors because of the requirement for continuous hand pressure to close the main switches.
Accordingly a need exists for a manually-controlled electric motor starter and overload protection system which completely eliminates all electromagnetic actuating circuitry and thereby renders the system more durable and reliable, requires less repair, removes all danger of high current leakage and resultant shock to the manual operator, is adaptable for both momentary and continuous running of motors and results in motor stoppage if a power failure occurs in the actuating circuitry.